Prime Minister John Key has carpeted one minister for getting involved in a potential conflict of interest and shot down another for suggesting a tax on plastic bags after being blindsided twice in 24 hours.

Mr Key said he had given a "bollocking" to the most serious offender, Internal Affairs Minister Richard Worth.

Dr Worth went to India on a private visit, sanctioned by Mr Key, but while he was there, he became involved in promoting an aviation company in which he had an interest.

It was reported yesterday that Dr Worth spoke in his capacity as a minister about the benefits of using New Zealand for aviation training.

Mr Key said he would not have sanctioned the trip if he had known Dr Worth was a director and shareholder in an aviation company which was in a joint venture with an Invercargill flight training academy.

Mr Key made it clear when he named his ministers that he wanted "outcomes, results and accountability". Yesterday, he said if anyone in government "needed a bollocking" it would come from him.

"I'm happy to administer it. Just ask Dr Worth," he said.

He said he expected ministers to work out potential conflicts of interest themselves, or to seek advice from the Cabinet Office if they were unsure.

"I don't think he deliberately tried to mislead me, I don't think he sought to make a pecuniary gain. But I do think he was unwise in the course of action he took and I don't think he should have taken that action."

It is not the first time an overseas trip has got Dr Worth into trouble - in 2002 in Egypt he opted out of official duties and went on a camel ride instead.

Asked if he would adopt a "three strikes and you're out" policy, Mr Key said: "I'm an extremely patient person, but there's a limit even to my patience."

Dr Worth had subsequently resigned as a director and shareholder of New Zealand Aviation and as chairman of the India Trade Group.

Mr Key's second target was Environment Minister Nick Smith, who floated a proposal to charge shoppers 5 cents for plastic bags.

Mr Key said there was no way he was going to support a charge that was in effect a tax going into the coffers of supermarkets. "My preference is to find a voluntary and industry-led solution," he said. "I've made that very clear to the minister."